This invention relates to the preparation of supports and catalysts which can be used to polymerize mono-1-olefins, such as ethylene.
Supported chromium catalysts have long been a dominant factor in the production of high density olefin polymers such as polyethylene. As originally commercialized, these catalysts were used in solution polymerization processes. However, it early became evident that a more economical route to many commercial grades of olefin polymers was a slurry process, that is, a polymerization process carried out at a temperature low enough that the resulting polymer is largely insoluble in the diluent. However, certain control techniques which are easily carried out in solution polymerization systems become more difficult in the slurry system. This is particularly true with regard to control of molecular weight. In a solution system, the temperature can simply be increased in order to provide a lower molecular weight, higher melt flow polymer. However, in slurry systems, there is a practical limit on temperature increases, since the point is quickly reached wherein the polymer goes into solution and thus the value of the slurry system is lost.
In order to allow maximum utilization of the more commercially desirable slurry system, modifications of the catalyst were developed so as to allow the production of higher melt flow polymer. One such modification is the formation of hydrogels comprising silica and other, optional, inorganic oxides. Unfortunately, these types of hydrogels can be costly and time consuming to prepare and, also, tend to have a low pore volume and high surface area. Furthermore, supported chromium catalysts do not always have a high activity, as expressed by grams of polymer produce by grams of catalyst per unit of time.